1. Field of the Invention
The invention concerns a generator of time-variable magnetic fields for a magnetic resonance device with at least one gradient coil, with conductors of the gradient coil extending substantially in the region of a hollow cylinder, and with the gradient coil being free of conductors in a middle region along the axial extent of the hollow cylinder, and a magnetic resonance device with such a generator.
2. Description of the Prior Art
Magnetic resonance technology is a known modality to, among other things, acquire images of the inside of a body of an examination subject. In a magnetic resonance device, rapidly switched gradient fields are generated by a gradient coil system and are superimposed on a static basic magnetic field that is generated by a basic field magnet. Furthermore, the magnetic resonance device has a radio-frequency system that radiates radio-frequency signals into the examination subject to excite magnetic resonance signals and acquires the excited magnetic resonance signals, on the basis of which magnetic resonance images are generated.
To generate gradient fields, appropriate currents are set in gradient coils of the gradient coil system. The amplitudes of the required currents are up to more than 100 A. The current rise and fall rates are up to more than 100 kA/s. Since the gradient coil system normally is surrounded by electrically conductive structures, eddy currents are induced in these by the switched gradient fields. Examples for such conductive structures are the vacuum vessel and/or the cryoshield of a superconducting basic field magnet. The fields arising as a consequence of the eddy currents are undesirable because, without counter measures, they weaken the gradient fields and distort them with regard to their time curve, which leads to impairment of the quality of magnetic resonance images.
The distortion of a gradient field as a result of the eddy current fields can be compensated up to a certain degree by a corresponding pre-distortion of a quantity used for controlling the gradient field. The eddy currents induced on a predetermined enveloping surface (that, for example, runs through an inner cylinder jacket of an 80-K cryoshield of the superconducting basic field magnet) by the gradient coils being fed with current also can be reduced by the use of an actively shielded gradient coil system. A gradient shielding coil associated with the gradient coil normally has a lower number of windings than the gradient coil, and is connected with the gradient coil such that the same current that flows through the gradient coil flows through the gradient shielding coil, but in the opposite direction. The gradient shielding coil thereby has a weakening effect on the gradient field in the imaging volume.
Furthermore it is known from German OS 3445724 to minimize magnetic coupling between an RF coil and a gradient field coil, for example by arranging shielding layers on both sides of the gradient field coil.
A magnetic resonance device is known from German OS 44 14 371 in which a radio-frequency shield is arranged between the radio-frequency antenna and the gradient coil system of the magnetic resonance device, the radio-frequency shield being permeable for the electromagnetic fields generated by the gradient coil system in the low-frequency range and impermeable for the fields generated by the radio-frequency antenna in the radio-frequency range. The radio-frequency shield has a first electrically conductive layer arrangement and a second electrically conductive layer arrangement arranged oppositely thereto that are separated from one another by a dielectric. The layer arrangements have adjacently arranged conductor tracks that are separated from one another by electrically insulted grooves; the grooves being offset in the first layer arrangement compared with the second; and in at least one of the layer arrangements, adjacent conductor tracks are connected with one another via specially arranged bridges, for example formed by capacitors, that conduct high-frequency currents.
The radio-frequency antenna of the magnetic resonance device may be fashioned as a so-called birdcage antenna. A birdcage antenna normally is fashioned to generate a homogenous radio-frequency field within a volume enclosed by it, with conductors that are parallel to one another and equally separated being connected with one another via ferrules and defining a cylindrical surface. Tuning is accomplished in the high-pass and low-pass filter ranges by capacitors in each of the conductors, or in the ferrules between the conductors, such that a homogenous radio-frequency field results upon resonance. Embodiments of such a birdcage antenna are found, for example, disclosed in U.S. Pat. No. 4,680,548. The radio-frequency antenna also can be fashioned as an array antenna. The array antenna is characterized by a number of essentially uniform, mutually overlapping conductor loops. Embodiments of such an array antenna are disclosed, for example, in U.S. Pat. No. 4,825,162.
A magnetic resonance device with a gradient coil system is known from German OS 101 56 770, in which an electrically-conductive structure is arranged and fashioned such that, at least within an imaging volume of the magnetic resonance device, a magnetic field of the structure caused by a gradient field via induction effects is similar to the gradient field. In an embodiment, at least one part of the structure is fashioned barrel-shaped as a component of a basic field magnet. Among other things, the gradient coil system can be fashioned without gradient shielding coils, since the undesirable results of the switched gradient fields (due to the similarity of the magnetic field caused by the structure) can be almost completely controlled by a pre-distortion, such that no weakening of the gradient fields occurs due to the gradient shielding coils.
An MR device is known from German OS 4230145 that has a basic field magnet that enables a transverse access to the measurement volume. The MR device has a gradient coil system with axially separated segments. To generate an essentially homogenous RF field in the measurement volume, an axial RF coil system is used that can be introduced into an axial bore of a supporting body or transversally into the recess of the basic field magnet. The MR device or, respectively, its components (such as the basic field magnet, gradient coil system and RF coil system) are fashioned to achieve an optimally large access to the measurement volume for simple implementation of therapy measures such as microsurgical operations, etc.
An MR device is known from U.S. Pat. No. 4,864,241 in which eddy currents are compensated. This ensues by the use of two-part gradient coils that typically form a hollow-cylindrical unit. For RF field generation, a likewise hollow-cylindrically fashioned RF antenna with smaller radius is introduced into the gradient coil unit. Such a design has the disadvantages that it requires a significant amount of space, and that the examination volume of the MR device is determined by the diameter of the RF antenna.